Key enzymes involved in retinoid metabolisms are alcohol and aldehyde dehydrogenases
that convert retinols to aldehydes and aldehydes to carboxylic acids, respectively. The
first oxidation reaction is catalyzed by a large number of enzymes from the
Dehydrogenase/reductase (SDR family), and by classic medium chain Alcohol dehydrogenases
[1].

Rhodopsin is converted by photoabsorption to
metarhodopsin, and the latter is reconverted to Rhodopsin by
light. It is well known that Rhodopsin can be formed from
opsin only when (11Z)-Retinal is present. The
photoisomerization of Retinal released during the
degradation of metarhodopsin is catalyzed by an unknown isomerase is and this
photoisomerization stereospecifically directed toward the formation of
(11Z)-Retinal [2], [3].
Retinal is also reduced in the reaction catalyzed by
all-trans-retinal-specific Retinol dehydrogenases.- Retinol
dehydrogenase 11 (all-trans/9-cis/11-cis) (RDH11), [1], [4], Alcohol dehydrogenase, iron containing, 1
(ADHFE1), [5], Dehydrogenase/reductase (SDR family) member 3
(DHRS3) [6], Retinol dehydrogenase 5
(11-cis/9-cis) (RDH5) [7], [8], [9], [10], [11], Retinol dehydrogenase 12
(all-trans/9-cis/11-cis) (RDH12) [12], [13], [14], retinol dehydrogenase 14 (all-trans/9-cis/11-cis)
(RDH14) [1], [15],
dehydrogenase/reductase (SDR family) member (RDH14) [16]. This dehydrogenase
activity utilizes [H+] of NADH and does not require NAD+ to generate
Retinol. These enzymes also catalyze oxidizing
(11Z)-Retinol with concomitant generation of [H] NADH to
complete the cycle.

Retinol is further isomerized via inversion of the C15
prochiral methylene hydroxyl group configuration resulting in formation of
(11Z)-Retinol. This reaction is catalyzed by specific
isomerase [17], [18].

Retinol can also esterification to format
Retinol palmitate and 11-cis-Retinyl
palmitate which can be either stored in the cell or processed further
[19]. The 11-cis-Retinyl palmitate can be
hydrolyzed at the rate ~20 times faster than Retinol
palmitate. Human retinal epithelium contains distinct activities that
hydrolyze 11-cis-Retinyl palmitate and Retinol
palmitate [19], [20], [21].

Two key enzymes involved in carotenoid metabolism are Beta-carotene
15,15'-monooxygenase 1 (BCDO) and Beta-carotene oxygenase 2
(BCDP). The first one cleaves
Beta-Carotene to form Retinal
[43], [44]. The second enzyme is responsible for the
unconventional cleavage of Beta-Carotene to form
Beta-apo-10'-carotenal and
Beta-Ionone [44], [45].

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